1. Field of the Invention
This invention relates to double helix coils or springs, preferably formed from thermostatic metal and, more specifically, to a method and apparatus for fabrication of such coils or springs.
2. Brief Description of the Prior Art
Double helix coils or springs, preferably of the type formed from thermostat metal, are known in the art. A double helix coil or spring is essentially a helical member wherein the helix is formed from a prior formed helically shaped member. In other words, a double helix is formed by forming a wire shaped member, which can be flat or circular, into a first helix about the wire axis and then forming the first helix into a second helix about the axis of the first helix. Such coils or springs effect a linear motion and a linear force in accordance with ambient temperature change.
The conventional method of manufacturing a double helix coil or spring is to torsionally wind a first helix with an extended tab on each end of the first helix for use in subsequent operations. A secondary operation, also by torsional means, uses one of the tabs for anchoring the first helix to an arbor while the tab on the opposite end of the first helix is gripped so that tension can be applied to produce the desired torsion while a second helix is being wound. A third operation usually follows to stretch the now formed double helix to the desired free length. The two tabs at opposing ends of the double helix member identified hereinabove are usually used for attaching other helix fabrication means thereto. The tabs are then removed to provide the completed double helix spring.
A problem inherent in the prior art method and apparatus for forming such double helix coils or springs is that the apparatus for forming the spring or coil requires hand feeding for each operation and an excessive number of operations, such as, for example, the formation and later removal of the tabs at opposing ends of the spring or coil to enable operation thereon to form the double helix coil. For example, the application of a "thermal spring" will usually be to replace a mono-metal spring when thermal force compensation is necessary, in which case, the two tabs of a conventional double helix spring or coil must be eliminated. To do so, a fourth and perhaps fifth operation may be necessary to cut off the two tabs in the conventional method of manufacturing.